А гугль ответил примерно следующее (на английском):
The word Horcrux may be comprised of "hor" or "hore" (old English/middle-English) meaning "dirt, evil, impurity" and "crux" or "crúce" (old English) meaning "container, pitcher(ful), jar" which would therefore mean "container of evil"
Horcrux comes from "Hor" which is an abbreviation of Latin "Horreum", (warehouse or barn) and which as a prefix can be related to words like "horrible"; and "crux" which may be related to the Latin word "crucis" (pain). So it gives an idea of "pain store" or a "store of essentials."
The etymology of the word seems to be this: a combination of “hors” from the French “dehors” meaning outside and “crux” meaning “essence.” Thus, a Horcrux is a device for keeping your soul (the essence) outside your body.
According to series author J.K. Rowling, the invented term Horcrux has no specific etymology, although fans have created their own largely based on the word's resemblance to the Latin crux, or “cross.”
В общем, вместо этимологии - сплошная задорновщина, но в одном можно быть уверенным точно: Кощеи, желающие бессмертия, думают примерно в одном направлении.
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